The present invention relates generally to subsea well apparatus for coupling a riser string to a floating vessel over a subsea well to which the string is run. More particularly, the invention relates to a diverter type coupling having a rotating insert with a bottom stripper rubber seal for sealing off on operating tools, such as kellies, drill pipes or tool joints inserted through the diverter coupling and into the riser string.
In general, diverter couplings are well known and a typical diverter coupling is disclosed in U.S. Pat. No. 3,791,442 issued to Watkins on Feb. 12, 1974. In addition, diverter couplings are also well known in which a rotating insert is provided for sealing to, and rotating in common with, the various operating tools inserted therethrough. Reference is made to pages 4262 and 4263 of the Regan Forge and Engineering Company section of the 1974-75 Composite Catalog.
A diverter with rotating insert typical of the abovementioned well known couplings is shown in FIG. 4. Referring to FIG. 4, the diverter coupling is shown generally at 10. The diverter coupling 10 includes a lower housing 12 for receiving a suitable riser string (not shown). The lower housing 12 is provided with a diverter outlet 14 for diverting low pressure formation gas encountered in top hole drilling or any gas or liquid accumulation in the subsea riser system. An upper housing 16 receives the insert housing 18. The insert housing 18 is sealed within upper housing 16 by way of an annular packing ring 20. The packing ring 20 seals the insert housing 18 to the upper housing 16 to prevent the escape of pressurized gases and liquids. Within the insert housing 18 is journalled the rotating insert 22. As is the usual practice, the rotating insert 22 is mounted within the insert housing 18 on roller bearings 24 and is provided with seals 26 and 28 to prevent the escape of pressurized gases and liquids from between the insert housing 18 and rotating insert 22. A drill pipe 30 is shown as it would be used in a typical operation utilizing the diverter coupling 10. The lower end of the drill pipe extends downward through the riser string to the sea floor while the upper portion of the drill pipe extends to the rotary operating table on the drilling platform.
An important aspect of diverter couplings is to provide a suitable seal around the drill pipe 30 during drilling operations and while the drill pipe is being raised or lowered. In order to seal off the drill pipe 30 to prevent escape of pressurized gases upwardly through the diverter coupling, a stripper seal 32 is commonly used. The stripper seal is an annular resilient rubber boot which is mounted fixedly on the rotating insert 22. It is designed to expand and contact within certain limits to seal off on the drill pipe and other operating tools to force pressurized gases out through the diverter outlet 14 rather than allowing them to escape up through the rotating insert 22.
The sealing characteristics of stripper seals when used with operating tools such as drill pipes having circular cross sections only, has proved adequate in the past for most drilling operations. However, problems have been experienced with providing adequate sealing with the stripper seal when it is used for sealing not only drill pipes, but when the stripper seal is also used to seal off operating tools having polygonal cross sections, such as kellies. When a stripper seal having an essentially circular opening is used to seal off a hexagon or square kelly, the sealing action of the stripper seal is not uniform, thereby increasing the likelihood of gas leakage, especially when high pressure gas accumulations are encountered. Further, the hexagon or square kelly tends to deform the usually circular inner sealing surface of the stripper seal to render it less effective in sealing off a circular drill pipe which is subsequently inserted into the diverter coupling. Since in many drilling operations, drill pipes and kellies are lowered and raised through the diverter coupling repeatedly, the stripper seal becomes deformed to prevent adequate sealing around the circular drill pipe, while at the same time providing a less than optimum seal around the non-circular kelly.
It is therefore desirable to provide a suitable means for protecting the stripper seal from contacting polygonally cross sectioned operating tools, such as kellies, during drilling operations. It is further desirable that while the stripper seal is being protected from contacting the kelly, that an alternative and more suitable means be provided for sealing the kelly to the rotating insert to prevent escape of gases or fluids from the diverter coupling.